1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a transition-metal hexacyanoferrate (TMH) cathode battery and associated fabrication processes.
2. Description of the Related Art
A battery is an electrochemical cell through which chemical energy and electric energy can be converted back and forth. The energy density of a battery is determined by its voltage and charge capacity. Lithium has the most negative potential of −3.04 V vs. H2/H+, and has the highest gravimetric capacity of 3860 milliamp-hours per gram (mAh/g). Due to their high energy densities, lithium-ion batteries have led the portable electronics revolution. However, the high cost of lithium metal renders doubtful the commercialization of lithium batteries as large scale energy storage devices. Further, the demand for lithium and its reserve as a mineral have raised the need to build other types metal-ion batteries as an alternative.
Lithium-ion (Li-ion) batteries employ lithium storage compounds as the positive (cathode) and negative (anode) electrode materials. As a battery is cycled, lithium ions (Li+) are exchanged between the positive and negative electrodes. Li-ion batteries have been referred to as rocking chair batteries because the lithium ions “rock” back and forth between the positive and negative electrodes as the cells are charged and discharged. The positive electrode (cathode) material is typically a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO2), or a material having a tunneled structure, such as lithium manganese oxide (LiMn2O4), on an aluminum current collector. The negative electrode (anode) material is typically a graphitic carbon, also a layered material, on a copper current collector. In the charge-discharge process, lithium ions are inserted into, or extracted from interstitial spaces of the active materials.
Similar to the lithium-ion batteries, metal-ion batteries use the metal-ion host compounds as their electrode materials in which metal-ions can move easily and reversibly. As for a Li+-ion, it has one of the smallest radii of all metal ions and is compatible with the interstitial spaces of many materials, such as the layered LiCoO2, olivine-structured LiFePO4, spinel-structured LiMn2O4, and so on. Other metal ions, such as Na+, K+, Mg2+, Al3+, Zn2+, etc., with large sizes, severely distort Li-based intercalation compounds and ruin their structures in several charge/discharge cycles. Therefore, new materials with large interstitial spaces would have to be used to host such metal-ions in a metal-ion battery.
Transition-metal hexacyanoferrates (TMHs) have been investigated as the cathode materials in lithium-ion batteries (LIBs) [1, 2] because they accommodate lithium-ion intercalation in their interstitial spaces. However, the lithium-ion size is too small to match the spaces, which degrades the TMH capacities rapidly during lithium-ion intercalation. In 2004, Eftekhari [3] used iron hexacyanoferrate (Prussian blue) as the cathode material in potassium-ion batteries (KIBs) with a counter electrode of potassium metal. The organic electrolyte was 1M KBF4 in 3:7 ethylene carbonate/ethylmethyl carbonate (wt.). The size of potassium-ion is almost two times that of the lithium-ions, and matches the interstitial spaces of Prussian blue very well. The results showed that Prussian blue was a good electrode material for KIBs, demonstrating a reversible capacity of ca. 75 mAh/g and a good capacity retention.
Similarly, Cui's group studied the intercalation behavior of large ions, for example, sodium, potassium and ammonium ions, in copper (CuHCF) and nickel hexacyanoferrates (NiHCF) with an aqueous electrolyte [4-6]. These large size ions were compatible with the interstitial spaces of the hexacyanoferrates, so that CuHCF and NiHCF demonstrated good capacity retention. Due to the narrow electrochemical window of water, these materials were evaluated under low voltages and demonstrated low energy density. In order to improve the performance, organic electrolytes with a wide electrochemical window would have to be used to increase the operation voltages of the TMH electrodes.
Goodenough's group [7] investigated a series of Prussian blue analogues in sodium-ion batteries (SIBs) with organic electrolytes. They found that KFe(II)Fe(III)(CN)6 demonstrated the highest capacity of 95 mAh/g, and KMnFe(CN)6, KNiFe(CN)6, KCuFe(CN)6, and KCoFe(CN)6 had a capacity of 50˜70 mAh/g. In the first 20 cycles, the capacity retention of KFeFe(CN)6 was higher than 97%.
FIG. 1 is a diagram depicting the crystal structure of a transition-metal hexacyanoferrate (TMH) in the form of AxM1M2(CN)6 (prior art). TMHs have an open framework. The large tetrahedrally coordinated A sites can host alkali, alkaline ions (Ax), and H2O molecules. The number of alkali or alkaline ions in the large cages of this crystallographically porous framework may vary from x=0 to x=2, depending on the valence of M1 and M2 that are metal ions. Of course, as the electrode materials in SIBs or KIBs, TMHs are expected to have two of Na+- or K+-ions in their interstitial spaces. Therefore, M1 and M2 with valences of +2 are selected in the synthesis process to produce (Na,K)2M1M2(CN)6. Moreover, M1 and M2 can be reversibly oxidized and reduced between the valences of +2 and +3 when Na+- or K+-ions are extracted/inserted from/into TMHs. When these TMHs are used as the electrode materials in SIBs or KIBs, it is hard to obtain very smooth and flat charge/discharge curves due to the fact that M1 and M2 have different chemical potentials, or occupy different spin states. For example, Na2CoFe(CN)6 exhibited two plateaus in its discharge curves at 3.78 V and 3.28 V vs. Na/Na+, that correspond to the reduction of Co3+ and Fe3+, respectively [8].
In order to obtain cheap electrode materials for batteries, manganese is a good choice for TMHs, e.g., (Na,K)xMn[Fe(CN)6]y.zH2O. Matsuda and Moritomo [9] synthesized a Na1.32Mn[Fe(CN)6].3.5H2O film for a lithium-ion battery that showed three plateaus in its discharge curve, and two plateaus in its charge curve that could be explained by the reduction and oxidation of Mn and Fe in the material. In the battery, the materials showed a capacity of 128 mAh/g. In a sodium-ion battery, Goodenough's group [7] also reported multiple plateaus in its charge/discharge curves. The Mn-based TMHs demonstrated a capacity of 70 mAh/g. However, it would be better to have a single, rather than two voltage plateaus in the charge/discharge curves. A battery with a single plateau charge/discharge curve has a tighter (more uniform) charge/discharge voltage than a battery with multiple plateaus. A tighter charge/discharge voltage renders a simpler battery control.
FIGS. 11A and 11B are graphs depicting the electrochemical behavior of a synthesized NaxMn[Fe(CN)6]y.zH2O cathode in sodium-ion batteries (prior art). When sodium ions electrochemically move in and out of the interstitial space of magnesium hexacyanoferrate (MnHCF), two main potentials appear in the charge or discharge process, due to the redox reaction of Mn and Fe [7, 9] in MnHCF. In battery applications, the redox reaction of MnHCF causes two plateaus during charge/discharge. Two plateaus are observed during charge/discharge that correspond to the redox reaction of Mn at higher voltages and the redox reaction of Fe at low voltages.                [1] V. D. Neff, Some performance characteristics of a Prussian Blue battery, Journal of Electrochemical Society, 132 (1985) 1382-1384.        [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, T. Yamagishi, Lithium intercalation, behavior into iron cyanide complex as positive electrode of lithium secondary battery, Journal of Power Sources, 79 (1999) 215-219.        [3] Ali Eftekhari, Potassium secondary cell, based on Prussian blue cathode, Journal of Power Sources, 126 (2004) 221-228.        [4] Colin D. Wessells, Rober A. Huggins, Yi Cui, Copper hexacyanoferrate battery electrodes with long cycle life and high power, Nature Communication, 2(2011) 550.        [5] Colin D. Wessells, Sandeep V. Peddada, Robert A. Huggins, Yi Cui, Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries. Nano Letters, 11(2011) 5421-5425.        [6] Colin D. Wessel's, Sandeep V. Peddada, M. T. McDowell, Robert A. Huggins, Yi Cui, The effect of insertion species on nanostructured open framework hexacyanoferrate battery electrodes, Journal of Electrochemical Society, 159 (2012) A98-A103.        [7] Yuhao Lu, Long Wang, John B. Goodenough, Prussian blue: a new framework for sodium batteries. Chemistry Communication, 48(2012) 6544-6546.        [8] J. F. Qian, M. Zou, Y. L. Cao, H. X. Yang, NaxMyFe(CN)6 (M=Fe, Co, Ni): A New class of cathode Materials for sodium Ion batteries, Journal of Electrochemistry (Chinese), 18(2012) 108-112.        [9] T, Matsuda, Y. Moritomo, Thin film electrode of Prussian blue analogue for Li-ion battery, Applied Physics Express, 4(2011) 047101.        
It would be advantageous if a TMH cathode battery could be made to operate with a single plateau charge and discharge curve.